Since insects are vectors for many parasitic diseases of humans and animals, understanding the interactions between parasites and their insect hosts is an important area of study. Critical to the development of many endoparasites is abrogation or evasion of the host immune response. However, the mechanisms by which parasites overcome insect immune responses remain obscure. We propose to examine the mechanism by which a parasite compromises the host immune system in a unique insect model system. We have found that the polydnavirus from the parasitic wasp Microplitis demolitor suppresses the encapsulation response of the wasp's lepidopteran host, Pseudoplusia includens. The presence of the virus is essential to the survival of the developing parasite, because the wasp is quickly encapsulated by the host if the virus is absent. Physiological studies suggest the host's inability to encapsulate the parasite is associated with alterations in specific types of blood cells (hemocytes). Nucleic acid hybridization experiments indicate that M. demolitor polydnavirus (MdPDV) DNA is present in host hemocytes, and MdPDV specific transcripts are detectable in hemocytes within 4 h of injecting MdPDV into the host. Further, higher levels of MdPDV expression are associated with hemocytes than any other host tissue. The objectives of this proposal are fourfold. First, we will characterize the persistence and expression of MdPDV in host hemocytes by Southern, Northern and in situ hybridization. Second, we will identify and characterize MdPDV RNAs from infected P. includens hemocytes using our already established MdPDV genomic library and/or a cDNA library approach. Third, we will determine whether venom from the adult wasp interacts with host hemocytes or in some way facilitates entry of MdPDV into host hemocytes. The interaction of venom with hemocytes and/or MdPDV will be characterized using antibodies already generated against venom components. Fourth, we will examine the physiological effects of MdPDV on the host immune response in vivo and in vitro. Attention will be focused on whether the altered response of hemocytes is due to factors present in plasma or to intrinsic alterations in hemocytes themselves. We also will determine whether the alterations in the host immune response are specific to the parasite or more general. The long-range goals of the study are to understand the basis by which insect parasites affect the host immune system and to gain insight into the invertebrate immune response.